In today's episode we’re going to space. Specifically, Mars. You didn’t really think that earth science is really limited to the earth, did you? Our topic today will be the Valles Marineris.
The Valles Marineris is a long series of canyons east of Olympus Mons, the largest mountain in the solar system. These canyons are about 4,000 km long, 200 km wide and up to 7 km (23,000 ft) deep. On terrestrial scales, the Valles Marineris is as long as the distance from New York to Los Angeles. That’s about the same as Beijing to Hong Kong or Madrid to Copenhagen for our international listeners. They are as wide as central Florida, central Italy, or the middle of the Korean peninsula. Two and a half times deeper than Death Valley, though only about 60 percent of the depth of the Marianas Trench, the lowest point on earth.
Not to be outdone, our planet, Earth, has even bigger valleys. These occur at the oceanic ridges, where plate spreading takes place. The longest rift valley on earth lies in the middle of the Mid-Atlantic Ridge, and it is more than double the length of the Valles Marineris. But let’s not belittle Mars. After all, while we have a pretty good idea for how oceanic rifts form on earth, there is quite a bit of debate about how Mars’ great valley formed.
The most popular theory suggests that the Valles Marineris are an analog to our oceanic rifts, and formed by the same process. As the volcanoes of the nearby Tharsis region developed, the Martian crust bowed down toward the center of the planet due to the weight of the new volcanic rocks. In time, the crust began to crack. This crack is what we see in the Valles Marineris. Unlike on Earth, this rift valley did not continue expanding, but shut down as the Tharsis Region, and Mars as a whole, cooled. Remember that unlike Earth, Mars does not have plate tectonics. It doesn’t have a continual process of hot material (like lava) rising to the surface, while relatively cold material (like the oceanic crust) is brought down towards the planet’s center.
More recent work has used satellite images, and high resolution elevation data to develop new insight into how the Valles Marineris formed. While images from the 1970’s Mariner 9 orbiter were quite blurry by today’s standards, new missions in the late 90’s to early 2000’s have given us a better view of the Martian surface than we have available for the earth. The Mars Reconnaissance Orbiter can take images where each pixel is about 0.5 m or 20 inches. That is, the color on each image is an average of an area of 0.25 square meters, or 2.5 square feet. It can then use image pairs to estimate the elevation of any point on the Martian surface with a pixel size of 0.25 m, or about 10 inches.
These new satellite images include multispectral data, or images that look at different wavelengths of light. The camera on your phone works in the same way: There are sensors that pick up, red light, green light, and blue light. Your phone records the intensity of each color in each part of the image, and then plays it back on your phone’s screen to create a picture.
Some of the satellites orbiting Mars take this to the next level. They don’t just take different slices of colored light, but also longer wavelength, infrared light. If you’ve ever seen an image from a thermal imaging camera, you know what this is. Parts of you show up as hotter or colder on the screen. It’s the same with the surface of the earth, or Mars. Scientists can compare the intensity of different wavelengths of light from each point on the surface. They can then compare these values, with what would be expected for different rock types. In other words, we’re able to roughly determine the types of rocks on the Martian surface without ever setting a boot, or rover tread, on the red planet.
Data from these images has shown that the Valles Marineris have layered rock formations both on the sides of the canyons, and within them. The great valley has seen many landslides over the last 3.5 Billion years of its existence, as well as new and smaller canyons carved into it. Scientists now speculate that rather than just forming as a big crack in the Martian surface, the Valles Marineris have been sculpted by flowing water, either in its liquid form as rivers, or in its solid form as glaciers.
An alternative hypothesis proposes that the Valles Marineris formed as a crack during a massive, planetary scale landslide. This landslide was about half the size of the US or China. How do you form a landslide that big? Well, you need a large pile of relatively weak rock, and high elevations for the landslide to flow from.
A key player here is salt. Salt is relatively weak as compared to rock, and can deform easier when squeezed. It can also hold water, which can be driven off by heating. On Earth, weak salt layers are partly responsible for undersea landslides in the Gulf of Mexico. The Opportunity rover had found some salt layers during its mission on Mars, so we know salt is present on the red planet.
Some scientists interpret the layers on the sides of the Valles Merinaris to be made of salt, and possibly include pockets of ice. This would imply that those layers are weak, and could potentially move downhill under the right circumstances.
Heating in the Tharsis region helped de-water salts under the future landslide, melted ice pockets, and created high elevations on one side of it. Think of it like putting a can on a wet metal sheet. If you raise one side of the sheet, the can will slide to the lower side. Just like that, the salty Martian crust broke, and slid downhill.
A crack in the side of this landslide allowed massive amounts of underground water to escape. As the water flowed downhill, it eroded the crack to form a massive canyon. This canyon is the Valles Marineris. The flood that helped form the Valles Marineris was probably bigger than any seen on earth. Bigger than the massive glacial outburst floods that formed the channeled scablands of the northwestern United States. Dick Gibson discussed outburst flooding in the December 27, 2014 episode. Unlike the Earth, the Martian surface has been relatively quiet since the Valles Marineris formed 3.5 billion years ago.
This episode was recorded at the studios of KBMF-LP 102.5 in beautiful and historic Butte, Montana. KBMF is a local low-power radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.